Formula & Calculator
Pregnancy Due Date (Naegele's Rule)
Standard clinical rule for estimating a baby's due date from the first day of the mother's last menstrual period.
Interpretation
Due date = Last menstrual period + 280 days. Estimates expected delivery date. Used in obstetrics for pregnancy dating and planning.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Due date | Estimated delivery date | |
| LMP | Last menstrual period date |
What it means
Naegele’s rule estimates the due date for a pregnancy based on the first day of the last menstrual period (LMP). By adding 280 days (40 weeks) to the LMP, it gives an approximate delivery date. It assumes a regular 28‑day cycle and ovulation on day 14. While simple and widely used, it has limitations: cycle variations affect accuracy. In modern practice, ultrasound dating is more precise. It is used in prenatal care to monitor fetal growth, plan tests, and manage pregnancy. Understanding this rule is essential for expectant parents and healthcare providers to track pregnancy milestones.
Worked example
Pregnancy Due Date (Naegele's Rule) – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| LMP | Jan 1 |
| Parameter | Value |
|---|---|
| LMP | Jun 1 |
Common mistakes
- Naegele’s rule: Due date = Last menstrual period (LMP) + 280 days.
- Assumes: A 28‑day cycle with ovulation at day 14 – not all women have regular cycles.
- Adjustment: If cycles are longer or shorter, adjust the due date accordingly.
- Calculations: Can also be done by subtracting 3 months from LMP and adding 7 days.
- Ultrasound: Early ultrasound dating is more accurate – this is a rough estimate.
Applications
Naegele's rule estimates the due date of a pregnancy by adding 280 days (40 weeks) to the first day of the last menstrual period. This is the standard method used in obstetrics for pregnancy dating, though it assumes a regular 28‑day cycle and ovulation on day 14. Obstetricians and midwives use it to establish an expected delivery date, to schedule prenatal visits, and to guide pregnancy management. By knowing the due date, healthcare providers can monitor fetal growth, plan delivery interventions, and reassure expectant parents. While not always exact, it is a practical starting point for pregnancy care. Understanding Naegele's rule is essential for maternity care and pregnancy planning.
- Establishment of gestational age and expected delivery date
- Planning of prenatal care and screening tests
- Fetal growth monitoring and intervention timing
- Maternity leave planning for mothers
- Education for expectant parents and health literacy
Frequently Asked Questions
It is a standard clinical method to estimate a baby's due date from the first day of the mother's last menstrual period (LMP): Due date = LMP + 280 days.
It represents the average gestation period (40 weeks) from the first day of the last menstrual period, which assumes a 28‑day cycle with ovulation on day 14.
Add 7 days and subtract 3 months from the first day of the LMP. For example, LMP = 1 January → due date = 8 October.
- Assumes a regular 28‑day cycle; cycle length variations shift ovulation and conception.
- Only about 5% of babies are born exactly on the predicted due date.
- It is less accurate for women with irregular cycles or who ovulate late.
They are essentially the same; EDC is the predicted delivery date. Naegele's rule gives the EDC.
LMP = 15 March. Add 280 days: 15 March + 280 days = 20 December (approximately).
Ultrasound in the first trimester is more accurate for dating, especially if the cycle is irregular. It can adjust the due date.
- Assuming the rule gives an exact delivery date.
- Applying it to women with irregular cycles without adjustment.
- Confusing LMP with conception date (conception occurs about 14 days later).
Yes, it is a quick initial estimate, but ultrasound dating is preferred for clinical management.
It helps schedule prenatal visits and interventions, though it is not a strict deadline for delivery.